6S> LIGHT AND THE BEHAVIOR OF ORGANISMS 



However this may be, It must be conceded that while the 

 results of these experiments indicate that orientation is due 

 to diversity of light intensity on the reacting organ, they 

 do not definitely settle the question. 



Much more convincing results were obtained toward the 

 close of the work when it occurred to me that it would be 

 possible to prevent the bending toward the glower entirely, 

 without vitiating the results, by reflecting the beam of 

 light and illuminating the surface directed away from the 

 glower as well as that facing it. A small mirror of finest 

 quality 5 mm. X 2 cm. was therefore supported in the beam 

 of light in a vertical position 3 cm. from the plumule. By 

 careful manipulation and frequent adjustment it was pos- 

 sible to keep the intensity on the surface directed toward 

 the glower and the one opposite nearly the same, while the 

 difference of Intensity on the right and left sides was nearly 

 twice as great as it was when the beam was not reflected. 

 The reactions of 4 plumules of Zea mays were studied under 

 these conditions. All deflected definitely toward the more 

 highly Illuminated side, as represented In Fig. 7. These 

 results seem to prove conclusively that orientation In plu- 

 mules of the gramineae (grasses) is in some way regulated 

 by difference in light Intensity on opposite sides, and that the 

 direction in which the rays enter the tissue Influences the 

 direction of motion only in so far as this may produce 

 unequal illumination of different parts of the sensitive 

 tissue. 



A number of experiments were made with young nas- 

 turtium (Tropaeolum) leaves In graded light. Different 

 parts of the leaf blades were thus subjected to different 

 intensities. In some experiments one-half of the blade was 

 entirely in the shadow. I was unable to detect any influ- 

 ence of the unequal illumination of the blade on orientation. 

 The leaves turned toward the source of light just as they 

 did when the blades were entirely Illuminated by light of 

 equal Intensity throughout. The circumnutatlon move- 

 ments in these leaves were so great, however, that it would 



